Corydoras Catfish
A bottom dweller that shoals, so six is the starting number rather than one. It gulps air at the surface and takes oxygen from it through its gut.
Photo by Soulkeeper, CC BY 2.5
Overview
Corydoras are small armoured catfish from South America, up to about 7.5 cm, and they breathe air through their gut. FishBase records that they surface to take air between 1 and 45 times an hour, absorbing oxygen through the intestine, which is an adaptation to warm, still water with very little oxygen in it.
That single fact settles two questions about keeping them. The surface has to stay reachable, and a corydoras darting up to the top is doing something normal rather than gasping.
Temperament
Busy, harmless and social. They spend the day working over the substrate with their barbels and they are noticeably more active in a group. Wild shoals run to twenty or thirty fish, and five is the smallest number that behaves like a shoal in a tank.
They are nocturnal by preference, so a tank with dim evening light gets the most out of them. Nothing about a corydoras is territorial, and they ignore other fish almost completely.
Care and grooming
Sinking food, since they will not compete at the surface. Worms, crustaceans, insects and plant matter make up the wild diet, and sinking pellets with frozen food a few times a week matches it closely enough.
The substrate is the health issue. Barbels are how a corydoras finds food, and sharp gravel wears them down until it cannot. Soft sand or smooth rounded gravel keeps them intact. Water at 25 to 28 degrees C, which is 77 to 82 F, and pH 6.0 to 8.0.
Living and exercise
A 60 cm tank, roughly 20 gallons, with an open sand bed and shade at one end. Floor space matters more than height because they use the bottom and almost nothing else.
They mix with nearly anything that will not eat them, which makes them the standard bottom half of a community tank. Tank mates that dominate feeding time are worth watching, since a corydoras will not push in and can quietly go hungry in a tank that looks well fed.
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